I used a drill successfully without backing the plastic for support and without any cracks I also sanded the holes after to smooth out any rough edges. As for the amount of holes, it was trial and error- started with a few and let the humidity stabilize then added a few more on so on until the humidity was sitting steady at 60%. I had to drill quite a few holes (26L tub), about 8 on the long sides, 4 on the short sides, and 8 on the lid. I'm not sure what size the bit was though. Hope this helps!